12 PRACTICAL TREE REPAIR 



grip the soil more firmly every year, so they, too, 

 must grow. 



Now growth can take place in two ways. A 

 thing can expand or grow from within simultane- 

 ously in all its parts, as a dry sponge grows when 

 it is dropped in water, or it can grow by addition 

 to its outer surface — be built up. Some plants, 

 such as corn and the palms, grow in the first way ; 

 the rest, including all the northern woody plants, 

 grow in the second way. The second is obviously 

 the better way for the development of a frame- 

 work combining economy with strength and easy 

 " grow-ability," because the light and strong tis- 

 sues are so firmly woven and cemented together 

 that expansion, or growth throughout the stem, 

 is practically impossible. The trunks, branches, 

 and roots of our trees, then, are growing each year 

 by the addition of a layer, completely covering 

 them, of new wood. 



The frame of a tree not only supports the 

 foliage but also acts as a circulation medium be- 

 tween the roots and the leaves. The roots absorb 

 water and mineral salts from the soil, and this 

 crude fluid passes up to the leaves, where a small 

 part of the oxygen and hydrogen in the water are 

 combined with carbon dioxide taken from the air, 

 and where, by use of the energy of the sun's 

 light, the green " chlorophyll bodies " in the leaves 

 combine these simple elements into complex com- 



